Thatch, a layer of organic matter including tightly intermingled dead and living leaves, stems, and roots, often develops between the soil surface and the green vegetation and, left untreated, can result in deterioration of the turf quality. Thatch includes stolons, rhizomes, roots, crown tissue, leaf sheaths and blades. Thatch layer intermixed with sand or soil is known as mat layer. The mat layer generally lies below the thatch layer where sand or soil is intermingled with thatch due to cultural practices like core aeration and top dressing.
High organic matter accumulation in the form of thatch or mat causes depletion of oxygen and decreased saturated hydraulic conductivity and increased water content. This makes the turf more susceptible to drought, cold, insects, diseases and other problems such as welt wilt, soft surface, black layer, limited rooting etc. Thatch control thus represents a challenge in turf management.
Management of turfgrass greens includes monitoring and control of the formation of thatch and mat layers. A cause of problems in the thatch-mat layer includes the rapid change in the nature of organic matter from the structured organic matter in live plant root tissues to the unstructured organic matter in dead plant tissues. Although live organic matter is not supposed to adversely affect soil's physical properties, the dead gelatinous organic matter in thatch swells in the presence of water during decomposition and plugs the soil macropores (air-filled pores), causing low oxygen levels in the root zones.
Extensive root death during persistent wet and hot conditions plugs the air-filled pores causing a decrease in the infiltration rate as well as oxygen stress. Increased accumulation of organic matter causes anaerobic conditions, which further slows the rate of organic matter decomposition. Grasses generally produce more adventitious roots (surface roots) during anaerobic conditions, further increasing organic matter content. Although a small amount of organic matter reduces surface hardness, moderates soil temperature extremes, increases the resilience and improves wear tolerance of turfgrass surface, excessive thatch and mat layers are undesirable in turfgrass. Unfortunately, control and management of thatch and mat layer buildup poses a challenge.
Cultural or mechanical practices like core-aeration, vertical mowing, grooming, and application of topdressing (such as sand) have been used to manage the thatch-mat buildup but have not proven sufficiently effective. These cultural practices are also intensive in terms of cost, energy, and labor, and some may cause adverse effects on turfgrass quality and site-use for a period of time.
Thatch-mat layer results due to a more rapid rate of organic matter accumulation than degradation. It is believed that the rate of thatch degradation, and most microbial degradation mechanisms, are restricted by the presence of lignin, a plant cell wall constituent that is resistant to microbial degradation. Lignin is a 3-dimensional amorphous polymer with a random and unorganized methoxylated phenyl propane structure that serves as a barrier in the cell walls to limit the accessibility to the more biodegradable plant materials, such as cellulose and hemicelluloses, by microbial degraders. Natural degradation of lignin is carried out in the environment by certain white-rot fungi which solubilize and mineralize lignin with the help of lignolytic enzymes thus exposing cellulosic materials for further bacterial degradation in the environment.
White rot fungi are recognized as one of the few active lignin degrading microorganisms found in the nature. Oxidative enzymes produced by fungi are able to attack the aromatic contents in lignin and produce free radicals, leading to degradation of lignin. White-rot fungi preferentially attack lignin over cellulose or hemicellulose in the wood tissue. This process of selective delignification exposes cellulosic materials for further bacterial degradation in the environment. Thatch is high in lignin, and, for this reason, turfgrass species high in lignin content are more resistant to decomposition.
Oxidative enzymes such as laccases, lignin peroxidases and manganese peroxidases produced by white rot fungi attack the aromatic components of lignin and contribute to its effective degradation. They have been used in the pulp and paper industry to remove lignin from wood pulp. Laccases, the multi copper oxidases, act on a wide variety of aromatic compounds by reducing oxygen to water.